Fibers crosslinked in substantially individualized form and various methods for making such fibers have been described in the art. The term "individualized, crosslinked fibers", refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between cellulose molecules of separate fibers. Individualized, crosslinked fibers are generally regarded as being useful in absorbent product applications. The fibers themselves and absorbent structures containing individualized, crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncrosslinked fibers. Often, the improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized crosslinked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncrosslinked fibers. The term "resilience" shall hereinafter refer to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition. Wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition. For the purposes of this invention and consistency of disclosure, wet resilience shall be observed and reported for an absorbent structure moistened to saturation.
In general, three categories of processes have been reported for making individualized, crosslinked fibers. These processes, described below, are herein referred to as dry crosslinking processes, aqueous solution crosslinking processes, and substantially non-aqueous solution crosslinking processes.
Processes for making individualized, crosslinked fibers with dry crosslinking technology are described in U.S. Pat. No. 3,224,926, L. J. Bernardin, issued Dec. 21, 1965. Individualized, crosslinked fibers are produced by spraying cellulose drylap with crosslinking agent, defiberizing the fibers by mechanical action, and drying the fibers at elevated temperature to effect crosslinking while the fibers are in a substantially individual state. The fibers are inherently crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. Processes as exemplified in U.S. Pat. Nos. 3,224,926, wherein crosslinking is caused to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for making "dry crosslinked" fibers. Dry crosslinked fibers are generally highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience. Dry crosslinked fibers are further characterized by low fluid retention values (FRV).
Processes for producing aqueous solution crosslinked fibers are disclosed, for example, in U.S. Pat. No. 3,241,553, F. H. Steiger, issued Mar. 22, 1966. Individualized, crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on cellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in an uncollapsed, swollen state. Relative to dry crosslinked fibers, aqueous solution crosslinked fibers as disclosed in U.S. Pat. No. 3,241,553 have greater flexibility and less stiffness, and are characterized by higher fluid retention value (FRV). Absorbent structures made from aqueous solution crosslinked fibers exhibit lower wet and dry resilience than structures made from dry crosslinked fibers.
In U.S. Pat. No. 4,035,147, Sangenis et al., issued Jul. 12, 1977, a method is disclosed for producing individualized, crosslinked fibers by contacting dehydrated, nonswollen fibers with crosslinking agent and catalyst in a substantially nonaqueous solution which contains an insufficient amount of water to cause the fibers to swell. Crosslinking occurs while the fibers are in this substantially nonaqueous solution. This type of process shall hereinafter be referred to as a nonaqueous solution crosslinked process; and the fibers thereby produced shall be referred to as nonaqueous solution crosslinked fibers. The nonaqueous solution crosslinked fibers disclosed in U.S. Pat. No. 4,035,147 do not swell even upon extended contact with solutions known to those skilled in the art as swelling reagents. Like dry crosslinked fibers, they are highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience.
Crosslinked fibers as described above are believed to be useful for lower density absorbent product applications such as diapers and also higher density absorbent product applications such as catamenials. However, such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers to result in significant commercial success. In addition, such fibers typically exhibit high objectionable odor and have low fiber brightness.
The use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is known in the art. See, for example, U.S. Pat. No. 3,224,926, Bernardin, issued on Dec. 21, 1965; U.S. Pat. No. 3,241,553, Steiger, issued on Mar. 22, 1966; U.S. Pat. No. 3,932,209, Chatterjee, issued on Jan. 13, 1976; U.S. Pat. No. 4,035,147, Sangenis et al, issued on Jul. 12, 1977; and U.S. Pat. No. 3,756,913, Wodka, issued on Sep. 4, 1973. Unfortunately, the irritating effect of formaldehyde vapor on the eyes and skin is a marked disadvantage of such references. In addition, such crosslinked fibers typically exhibit high objectionable odor and have low fiber brightness. A need is evident for cellulosic fiber crosslinking agents that do not require formaldehyde or its unstable derivatives.
Other references disclose the use of dialdehyde crosslinking agents. See, for example, U.S. Pat. No. 4,689,118, Makoui et al, issued on Aug. 25, 1987; and U.S. Pat. No. 4,822,453, Dean et al, issued on Apr. 18, 1989. The Dean et al reference discloses absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of C.sub.2 -C.sub.9 dialdehydes, with glutaraldehyde being preferred. These references appear to overcome many of the disadvantages associated with formaldehyde and/or formaldehyde addition products. However, the cost associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may be too high to result in significant commercial success. Therefore, there is a need to find cellulosic fiber crosslinking agents which are both safe for use on the human skin, good aesthetics (exhibit low odor, have high fiber brightness), and also are commercially feasible.
The use of specific polycarboxylic acids to crosslink cellulosic fibers is also known in the art. See, for example, U.S. Pat. No. 5,137,537, Herron et al., issued Aug. 11, 1992, U.S. Pat. No. 5,183,707, Herron et al., issued Feb. 2, 1993, and U.S. Pat. No. 5,190,563, Herron et al., issued Mar. 2, 1993. The Herron et al. references disclose absorbent structures containing individualized cellulosic fibers crosslinked with a C.sub.2 -C.sub.9 polycarboxylic acid. The ester crosslink bonds formed by the polycarboxylic acid crosslinking agents are different from the crosslink bonds that result from the mono- and di-aldehyde crosslinking agents, which form acetal crosslinked bonds.
Absorbent structures made from these individualized, ester-crosslinked fibers exhibit increased wet resilience and dry resilience and improved responsiveness to wetting relative to structures containing uncrosslinked fibers. Furthermore, the preferred polycarboxylic crosslinking agent i.e., citric acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde addition products. Unfortunately, the preferred C.sub.2 -C.sub.9 crosslinking agent, citric acid, can cause discoloring (i.e., yellowing) of the white cellulosic fibers when treated at elevated temperatures. In addition, unpleasant odors can also be associated with the use of alpha-hydroxy carboxylic acids such as citric acid. The Herron et al. references do not include processes by which to reduce the odor or increase fiber brightness.
It has now been discovered that the characteristic odor can be removed and the brightness improved by contacting the fibers with an alkaline solution (e.g., an aqueous solution of sodium hydroxide) and an oxidizing bleaching agent (e.g., hydrogen peroxide). The alkaline solution raises the finished fiber pH preferably to the 5.5-6.5 range from about 4.5. This in combination with the oxidizing bleaching agent eliminates the "smokey and burnt" odor characteristics of the crosslinked fibers. The oxidizing bleaching agent when added at high consistency increases the final product brightness to 80 to 86 from 70 to 75, and reduces odor.
It is an object of this invention to provide a process for preparing reduced odor and brighter individualized fibers crosslinked with a polycarboxylic acid crosslinking agent which have improved absorbency and aesthetic properties. Absorbent structures made from the individualized, polycarboxylic acid crosslinked fibers exhibit higher wet resilience and higher dry resilience than structures made from uncrosslinked fibers.
It is a further an object of this invention to provide individualized fibers crosslinked with a polycarboxylic crosslinking agent and contacted with an alkaline solution and oxidizing agent and absorbent structures made from such fibers as described above, which have a superior balance of aesthetic properties relative to prior known crosslinked fibers.
It is a further object of this invention to provide reduced odor and improved brightness individualized fibers crosslinked with a polycarboxylic crosslinking agent and absorbent structures made from such fibers, as described above, which have a superior balance of absorbency properties relative to prior known crosslinked fibers.
It is additionally an object of this invention to provide a commercially viable process for preparing reduced odor and brighter individualized, crosslinked fibers and absorbent structures made from such fibers, as described above, which can be safely utilized in the vicinity of human skin.